1. Field of the Invention
The present invention relates to a color thermosensitive printer that uses a color thermosensitive recording paper. The present invention relates more particularly to an optical fixing device for the color thermosensitive printer, that provides uniform optical fixation of the color thermosensitive recording paper while taking illuminance variation into consideration.
2. Background Arts
The color thermosensitive recording paper has at least three thermosensitive coloring layers, i.e., cyan, magenta and yellow thermosensitive coloring layers, formed on atop another on a base material. Among these coloring layers, the obverse coloring layer has the highest thermal sensitivity, so it develops color with the lowest heat energy. The deeper from the obverse, the lower the thermal sensitivity of the coloring layer, so it requires the higher heat energy for coloring the deeper coloring layer. To print a full-color image, heating elements of a thermal head are pressed onto the obverse surface of the recording paper, to record pixels of at least three colors sequentially from the obverse coloring layer. Prior to recording on the next thermosensitive coloring layer, coloring capability of the upper thermosensitive coloring layer is extinguished by exposing to rays of a wavelength range that is specific to that coloring layer, so the upper coloring layer would not develop color even though higher heat energy is applied for recording on the next coloring layer.
One known kind of color thermosensitive printer utilizes a thermal head that extends in a direction transverse to a paper conveying path, and records a line of pixels at a time on the color thermosensitive recording paper. In this type of printer, called a color thermosensitive line printer, one color frame of a full-color image, e.g. a yellow frame, is recorded line by line on the yellow coloring layer as the recording paper is moved along the paper conveying path in synchronization with the line recording of the thermal head. The next color frame, e.g. a magenta frame, is recorded line by line after the yellow thermosensitive coloring layer is optically fixed. After the magenta coloring layer is optically fixed, a cyan frame is recorded line by line. This method is called a three-color frame sequential recording. In many of this type of printers, linear tube lamps that extend transversely to the paper conveying path are used for projecting the optical fixing rays.
Although the linear tube lamp is inexpensive and is able to project a large quantity of light with high efficiency, illuminance of the lamp lowers in end portions of its glass tube adjoining its caps or bases. Therefore, it is difficult to expose the recording paper to the optical fixing rays uniformly across the width without a long linear tube lamp that extends sufficiently beyond the width of the recording paper.
There is another type of color thermosensitive printer, called a color thermosensitive serial printer, wherein a small thermal head is mounted on a carriage, and is scanned across the width of the recording paper (main scanning), while the recording paper is moved intermittently in a lengthwise direction relative to the thermal head (sub scanning). Because of the small thermal head, the serial printer can be more compact and less expensive than the line printer.
JPA 5-124352 discloses a color thermosensitive serial printer that records a full-color image in a three-color line sequential fashion. That is, the thermal head serially records yellow pixels along the width of the recording paper while scanning in a forward direction across the width of the recording paper. Thereafter the thermal head serially records magenta pixels while scanning in a reverse direction on the same line as the first forward scanning. Then, the thermal head serially records cyan pixels on the same line as the yellow and magenta pixels while scanning again in the forward direction. After pixels of the three colors are recorded on the same line, the recording paper is advanced by one line, and three-color pixels are recorded on the next line in the same way as above.
To fix the previously recorded pixels, a small optical fixing lamp for yellow is disposed on the carriage behind the thermal head with respect to the forward scanning direction, and is turned on during the thermal recording for yellow and magenta. On the other hand, a small optical fixing lamp for magenta is disposed on opposite side of the thermal head from the yellow fixing lamp, i.e., behind the thermal head with respect to the reverse scanning direction, and is turned on during the thermal recording for magenta and cyan. Because of this configuration, time efficiency of thermal recording and optical fixing is remarkably improved.
However, the small optical fixing lamps are expensive because they are of special type. Due to their small size, the luminous intensities of these lamps are so small that it takes more time to apply a sufficient quantity of light enough for optical fixing. Besides that, since the lamps as well as the thermal head are mounted on the scanning carriage, complicated wiring is necessary for supplying and controlling them.